Chloride, one of the most prevalent anions found in water, combines most commonly with the cations sodium, calcium, and magnesium.
Chloride levels in most waters range from 10 to 100 mg/l, and sea water contains over 30,000 mg/l chloride in combination with sodium, as NaCl.
Chloride is an essential electrolyte that helps to maintain pH, transmit nerve impulses and regulate cellular fluids.
Chloride in water is more a plumbing issue than a health issue.
Chloride, when concentrated, can cause corrosion of metal piping, so when treating water high in chloride plastic is usually preferred to stainless steel for reverse osmosis membrane housings. Iron is leached into water from metal pipes when high levels of chloride are present. Chloride is the main cause of pitting of stainless steel. Chloride combines with hydrogen to produce hydrochloric acid.
The suggested MCL for chloride is 250 ppm. Above this level water often has an unpleasant salty taste.
Reverse osmosis removes around 95% of chloride, and electrodialysis and distillation are also effective. In industrial settings, strong base anion exchangers can be used.
In practical terms for most residential users, in city water chloride is not a problem. For well owners with high chlorides, undersink reverse osmosis takes care of drinking water. If water is so high in chlorides that it is unusable for irrigation, whole house reverse osmosis is an option.